I. Field of the Invention
This invention relates generally to dispatch systems and, more particularly, to the implementation of a dispatch system in a cellular system.
II. Description of the Related Art
In a wireless telephone communication system, many users communicate over a wireless channel to connect to other wireless and wireline telephone systems. Communication over the wireless channel can be one of a variety of multiple access techniques. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). The CDMA technique has many advantages. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307 issued Feb. 13, 1990 to K. Gilhousen et al., entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of the present invention and incorporated herein by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters, airborne repeaters, or terrestrial base station transceiver subsystems using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times permitting an increase in system user capacity.
In the CDMA cellular system, each base station transceiver subsystem provides coverage to a limited geographic area and links the remote units in its coverage area through a cellular system switch to the public switched telephone network (PSTN). When a remote unit moves to the coverage area of a new base station transceiver subsystem, the routing of that user's call is transferred to the new base station transceiver subsystem. The base station-to-remote unit signal transmission path is referred to as the forward link and the remote unit-to-base station signal transmission path is referred to as the reverse link.
In a typical wireless telephone communication system, the remote unit may employ a vocoding system which encodes voice information in a variable rate format. In a variable rate system, the data rate may be lowered due to pauses in the voice activity. The lower data rate reduces the level of interference to other users caused by the remote unit transmissions. At the base station a vocoding system is employed for reconstructing the voice information. In addition to voice information, data information alone or a mixture of the two may be transmitted by the remote unit.
When a remote unit is producing its own data for transmission, a internal vocoder produces from digital samples of the voice information encoded data at four different rates, e.g. approximately 8,000 bits per second (bps), 4,000 bps, 2,000 bps and 1,000 bps, based on voice activity during a 20 millisecond (ms) frame. Each frame of vocoder data is formatted with overhead bits as 9,600 bps, 4,800 bps, 2,400 bps, and 1,200 bps data frames. The highest rate data frame which corresponds to a 9,600 bps frame is referred to as a "full rate" frame; a 4,800 bps data frame is referred to as a "half rate" frame; a 2,400 bps data frame is referred to as a "quarter rate" frame; and a 1,200 bps data frame is referred to as an "eighth rate" frame. A vocoder which is suited for application in this environment is described in U.S. Pat. No. 5,414,796, entitled "VARIABLE RATE VOCODER," issued May 9, 1995 and assigned to the assignee of the present invention. When the remote unit receives data from an outside source such as a terminal equipment unit, the remote unit continues to process the data in this variable rate frame format.
When the original cellular telephone spectrum licenses were issued by the government, one of the restrictions on use of the spectrum was that the carriers could not provide dispatch services. However, because of the great advantages of the CDMA system and the inherent expense and problems of deployment and maintenance of private dispatch systems, the government is re-examining this issue. The government itself would benefit greatly from such services.
Whereas typical wireless and wireline telephone service provides point-to-point service, dispatch services provide one-to-many service. Common usage of dispatch services are local police radio systems, taxicab dispatch systems, Federal Bureau of Investigation and secret service operations, and general military communication systems.
The basic model of a dispatch system consists of a broadcast net of users. Each broadcast net user monitors a common broadcast forward link signal. If a net user wishes to talk, he presses a push-to-talk (PTT) button and he is granted system talker privileges. Typically the talking user's voice is routed from the reverse link over the broadcast forward link. Ideally the dispatch system allows landline and wireless access to the system. When the remote unit user has finished speaking, he releases the PTT button. In response the remote unit generates a push-to-talk off indication which terminates the privileges and frees the system for use by other system users.
If the push-to-talk button on a remote unit becomes stuck down, the remote unit may be granted system talker privileges. System resources are thus expended and other remote units may be prevented from accessing the system because the remote unit with the stuck push-to-talk button is blocking the system. This type of scenario is referred to as system deadlock and it is, of course, a highly undesirable state. The present invention is a method and apparatus for detecting system deadlock and truncating its deleterious effects.